Soft gels derived from a polymer containing a plurality of urethane linkages (a polyurethane) and a high level of plasticizer are well known in the art. Such gels have been used as potting compositions for electrical and telecommunications equipment, as energy absorbing material, as cushioning materials, and in many related applications. Further examples of specific applications of such materials include shoe inserts (such as arch supports), bicycle seat cushions, computer mouse pads, ergonomic elbow and wrist supports, helmet linings, and the like. Cushioning and ergonomic applications represent a growth area for these gel materials because these materials can be formulated to a consistency very similar to human fat. Such a consistency is ideal for supporting human body members that come into prolonged contact with an otherwise hard surface, such as the edge of a computer keyboard or the core of a bicycle seat. For similar reasons, these gels are finding a range of applications in medical applications and in items of personal protective equipment.
Highly plasticized gels have viscoelastic properties well suited to impact protection. The soft gels are generally solid gels, but are sometimes foamed to a modest degree to produce microcellular soft elastomers. The polyurethane gels are often, although not always, used behind a layer of fabric or an elastomeric film material. They are sometimes completely encapsulated by one or more such flexible facing (or backing) materials. Sometimes these encapsulated polyurethane gels are not solid, but instead may be flowable (albeit highly viscous) liquids. In this special situation, the encapsulating structures retain the gel in place.
Polyurethane gels are typically formed from the reaction of a polyfunctional organic isocyanate with a polyfunctional isocyanate reactive material in the presence of a non-volatile inert liquid. The polyurethane component of the gel is typically crosslinked (thermoset) and the isocyanate reactive material contributes flexibility. The polyurethane component of the gel, under the most preferred circumstances, couples to the non-volatile organic liquid by secondary bonding forces, such as hydrogen bonding and Van Der Waals interactions, in order to form a completely compatible plasticized gel in which the non-volatile liquid component(s) are bound and do not migrate or exude during use. The isocyanate reactive materials typically consist predominantly (on a weight basis) of flexible polyols known in the art. These polyols have equivalent weights greater than 500, typically about 1000 or greater, and nominal isocyanate reactive group functionalities of 2 to 4. The isocyanate reactive materials commonly used are polyether or polyester polyols. Aliphatic polyethers based on propylene oxide, sometimes in combination with ethylene oxide, are highly preferred. Nominal diols and triols are particularly preferred in the conventional gel systems, and mixtures of these are sometimes used. The polyols contain predominantly primary or secondary hydroxyl groups or combinations thereof. Typical gel formulations may also contain relatively low levels of low molecular weight chain extenders and/or crosslinkers known in the art. Examples of typical polyisocyanates used in making polyurethane gels include both aromatic and aliphatic polyfunctional isocyanates. The isocyanates of the MDI and TDI series are very widely used.
The loading of the non-volatile inert liquid in polyurethane gels is typically quite high. It is almost always higher than 10% by weight of the total gel composition, and is typically higher than 30% by weight of the total gel composition. Plasticizer loadings of greater than 50% of the total composition are well known. Plasticizers (typically inert, non-volatile liquids) that have been used in the past in preparing polyurethane gels include phthalate plasticizers (such as DIOP), vegetable oils, mineral oils, liquid resins such as polybutene resins, other kinds of ether and ester containing liquids, mixtures of these, and the like.
One of the oldest and most serious problems encountered in formulating polyurethane gel systems with high plasticizer loadings is overcoming the tendency of the plasticizer to separate, or migrate, out of the gel onto the surface thereof. Such migration, if severe enough, degrades the gel-like properties of the material over time. It can also create problems with staining due to the presence of excessive amounts of oily liquid on the surface of the gel. Many techniques have been used in the past to increase the compatibility of the plasticizer with the polyurethane in the gel. These techniques have included the use of special mixtures of plasticizers, such as mixtures of oils and phthalate plasticizers. The ester containing phthalate plasticizers are sometimes called “coupling agents” because they are believed to be capable of improving the compatibility of oils, such as mineral and vegetable oils, with the polyurethane component of the gel. It has been observed that certain ester containing plasticizers, particularly the popular phthalate plasticizers, can cause reduced cure rates in the reactive polyurethane portion of the gel formulation. The reasons for this retardation of cure rates is not altogether clear, but may be related to the presence of traces of acid in the ester based plasticizers. Complete elimination of all residual acidic species in ester containing plasticizers is very difficult and expensive. As a result of the retarding effect of these prior art plasticizers, higher loadings of urethane catalyst must be used. This adds to the cost of the system, and it would therefore be desirable not to have to add extra catalyst.
Petroleum based oils with a high aromatic content tend to have good compatibility in polyurethane gels and with the chemical precursors thereof (especially isocyanates). Unfortunately, most of these aromatic oils have serious issues regarding toxicity. To be sold in the United States, such oils must typically carry a cancer suspect agent warning. Inclusion of such oils into polyurethane gels is therefore problematic from a safety standpoint, especially if the gels are likely to come into contact, directly or indirectly, with people. Such is likely to be the case in many of the growth applications of polyurethane gels that were noted above. It should also be noted in this context that phthalate type plasticizers have come under recent scrutiny for possible toxic hazards. Therefore, it would be desirable not to have to use phthalate plasticizers.
There is accordingly a strong need in the industry for polyurethane based gel systems that do not contain plasticizing agents that exude from the gel, migrate, cause staining or odors, cause cure problems, and/or require a cancer suspect label or any other serious toxicity warnings. There are ongoing efforts to develop better plasticizers that meet the foregoing requirements. However, a better approach to meeting the needs of the industry is to develop a polyurethane gel system that eliminates the use of plasticizers entirely, or at least reduces the amounts required.
The prior art contains several references to the use of mono-ols as ingredients in elastomeric gel formulations or viscoelastic foams used in combination with polyols. Varying degrees of success have been reported with this strategy. However, the mono-ol based polyurethane gel formulations as a class tend to exhibit slow cure and may therefore be difficult to process. Gel formulations with high plasticizer levels tend to exhibit this problem as well.
The ideal reaction profile for a polyurethane gel formulation has a long gel time (or working time) and a short cure time. These requirements tend to be very difficult to satisfy simultaneously. Therefore, there is a need in the industry for a reaction system that is suitable for the preparation of elastomeric polyurethane gels without high levels of additive plasticizers, while offering a combination of both a long working time and fast cure.